Method for the production of steel sheet coated with a fluororesin film

ABSTRACT

A method of producing a steel sheet coated with a fluororesin film by heat-fusing the fluororesin film onto a surface of the steel sheet, comprising heating the surface of the steel sheet to a temperature higher by 30-120 degrees C. than a melting point of the fluororesin of the fluororesin film, and contact bonding the fluororesin film onto the steel sheet surface between a pair of rolls, while a surface of a roll facing the fluororesin film, in the pair of rolls, is heated to a temperature lower by 100-210 degrees C. than the melting point of the fluororesin of the fluororesin film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for the production of a steel sheetcoated with a fluororesin film, and more particularly proposes atechnique for strongly applying a fluoro- resin film containing fluorineatom therein onto a surface of a steel sheet.

2. Background and Related Art

In general, the fluororesin is excellent in the properties such ascorrosion resistance, stain resistance, adhesion resistance, heatresistance, incombustibility and the like. Therefore, if a stainlesssheet or the like can firmly be covered with a film of this fluororesin,the resulting coated steel sheet possesses both the above propertiesinherent to the film and the properties inherent to the stainless steel(strength and workability).

The steel sheets coated with the fluororesin film having the aboveproperties can widely be applied to a toasting mold for baking breads,cakes, confectioneries and the like, food cooking tools and food workingtools, heating cook tools such as an inner plate of a microwave oven, aninner pot of a rice cooker, a top plate of a gas table and the like andfurther kitchen tool materials such as material for oven hood and so on.Therefore, it is desired to establish a technique for industriallyproducing the above steel sheets.

In this connection, materials obtained by coating stainless steel sheetor plated steel sheet with a fluororesin have hitherto been disclosed inJP-A-61-137534, JP-A-61-138567 and JP-B-63-11147. In JP-B-59-16836 andJP-B-59-16837 are disclosed materials obtained by coating Al-platedsteel sheet with a fluororesin. In JP-B-54-24434, JP-B-59-10304 andJP-A-63-126728 are disclosed materials obtained by coating Al or Alalloy body with a fluororesin film. In JP-A-63-168333 is disclosed amethod of coating a steel sheet with a fluororesin film through athermosetting adhesive.

According to these conventional techniques, however, the coating bymeans of the adhesive is difficult to achieve and also the strongcovering is difficult because the fluororesin film has low surfacetension and hence a strong non-adhesion property. Particularly, therehas not yet been developed an effective method of strongly applying ontothe surface of the steel sheet with respect to fluororesin filmscontaining two or more fluorine atoms per one molecule unit such asethylene tetrafluoride-perfluoroalkylvinyl ether copolymer resin (PFA),ethylene tetrafluoride-propylene hexafluoride copolymer resin (FEP) andethylene tetrafluoride-ethylene copolymer resin (ETFE) up to thepresent, so that the practical use of these films is actually delayed.

In this point, the inventors have already proposed in JP-A-5-162243 amethod wherein a mixture of fluororesin and heat-resistant resin ispreviously applied onto a surface of aluminum plated steel sheet, zincalloying plated steel sheet or stainless steel sheet and a resin film ofthe same kind is applied thereon and pressed through rolls, during whichthe interface between the film and the steel sheet is heated to atemperature just above a melting point of the film to weld to theundercoat resin layer.

However, this technique adopts a simple method of conducting heatpressing only by single heating, so that there are problems that theadhesion force between the film and the steel sheet becomes insufficientand that melt-breaking of the film and poor adhesion are caused becauseonly the surface temperature of the steel sheet is controlled.

It is a main object of the invention to adhere a fluororesin filmcontaining two or more fluorine atoms per one molecule unit and having apoor adhesion property onto a surface of a stainless steel sheets or thelike at a high adhesion force.

It is another object of the invention to adhere continuously throughroll pressing a fluororesin film containing two or more fluorine atomsper one molecule unit onto a surface of a metal without causing meltbreakage and poor adhesion.

SUMMARY OF THE INVENTION

As a method for achieving the object of the invention, in view of theproblem of the conventional technique that the melt breakage or pooradhesion of the film is brought about as the temperature of press rollbecomes too low or high, the invention proposes a method of producing asteel sheet coated with a fluororesin film by heat-fusing thefluororesin film onto a surface of the steel sheet, characterized inthat a surface of the steel sheet to be adhered with the fluororesinfilm is previously adjusted by heating to a temperature higher by30°-120° C. than the melting point of the fluororesin, while a surfaceof a roll facing the film in a pair of rolls for contact-bonding theresin film onto the steel sheet surface is adjusted to a temperaturelower by 100-210° C. than the melting point of the fluororesin.

In this method, it is preferable that a primer comprised of a mixedresin of fluororesin and thermal-resistant resin is previously appliedonto the surface of the steel sheet at a thickness of 3-15 g/m² as aweight after the drying prior to the contact-bonding of the fluororesinfilm to the steel sheet.

Furthermore, it is preferable that at least one heating among the steelsheet surface heating, the heating of primer-treated steel sheet surfaceand the roll surface heating is carried out by induction heating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing heat-fusion conditions (based on a meltingpoint of a film) in Examples of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention is characterized by the regulation of bothtemperatures, the steel sheet surface temperature and the surfacetemperature of the roll for pressing, without directing attention onlyto the regulation of the former. Thus, the resin film is contact-bondedat a fused state only in an interface to the steel sheet, so that thefluororesin film can continuously and stably be adhered to the surfaceof the steel sheet without bringing about the melt breakage and pooradhesion.

The method of the invention will be described in detail below.

In the method of the invention, it is required to adjust previously thesurface of the steel sheet (the primer-treated steel sheet in case ofthe primer treatment) to be coated with the resin film by heating to atemperature higher by 30°-120° C. than the melting point of thefluororesin just before the roll pressing at a state of piling thefluororesin film onto the surface of the steel sheet. For example, it isnecessary to heat the steel sheet to 340°-430° C. in case of PFA resinfilm having a melting point of 310° C. or 300°-390° C. in case of FEPresin film having a melting point of 270° C.

The reason why the surface temperature of the steel sheet is adjusted tothe above range is due to the fact that when the temperature of thesurface to be coated with the film does not reach a temperature higherby 30° C. than the melting point of the fluororesin, the surface of thefluororesin film to be applied is not fused and hence the adhesionbetween the film and the steel sheet surface or between the film and theprimer becomes insufficient. On the other hand, when the surfacetemperature exceeds a temperature higher by 120° C. than the meltingpoint of the fluororesin, the film itself is melted and broken in thecontact-bonding, or the possibility of degrading the adhesion propertythrough thermal decomposition of the primer resin in case of the primeris foreseen.

When the fluororesin film is contact-bonded to the thustemperature-adjusted steel sheet surface through a pair of rolls, it isrequired to adjust the surface of the roll facing the film to atemperature lower by 100°-210° C. than the melting point of thefluororesin.

The reason why the surface temperature of the roll facing the film inthe contact-bonding is adjusted to the above range is due to the factthat when the roll surface temperature exceeds a temperature lower by100° C. than the melting point of the fluororesin, the cooling of thefilm surface is lacking and the film is at a molten state to cause arisk of melt-breaking the film, while when the roll surface temperatureis less than a temperature lower by 210° C. than the melting point ofthe fluororesin, the film is rapidly cooled and the interface to thefilm is not fused to cause the poor adhesion.

Moreover, the surface temperature of the roll facing the steel sheetwhich does not directly contact with the film is not particularlyrestricted, but it is desirable that the temperature is kept in therange between the temperature by 100° C. lower than the melting point offluoro-resin film and that by 50° C. higher than the melting point ofthe fluoro-resin.

In this method, rubber rolls or the like having good wettability to thesteel sheet surface and elasticity are favorably used as a pair of rollsfor the contact-bonding. As a result, the rolls made from such amaterial are degraded by heat from the steel sheet, so that it isimportant that the roll itself should be avoided to use at a temperaturenot lower than 300° C. and is adequately cooled to maintain a level ofnot higher than 210° C.

In the method of the invention, as mentioned above, what extent thesurface temperature of the steel sheet or the like is selected withinthe above range is controlled by the type and thickness of the film,surface temperature of the roll for contact-bonding and the like. Forexample, when the steel sheet or the primer-treated steel sheet isheated by passing through a tunnel furnace and thereafter taken out fromthe furnace and the film is pressed through the rolls, it is required toheat it to a higher temperature previously, anticipating a temperaturedrop. According to experience, the temperature drop is dependent uponfactors such as sheet thickness, sheet passing rate, distance betweenfurnace and press roll and the like, but is usually 10°-50° C., so thatit is important to conduct the extra heating including the dropquantity.

In the method of the invention, the heating of the steel sheet may beusually carried out by using a hot air type tunnel furnace, but ispreferably conducted by using an electromagnetic induction heatingfurnace. Because, in case of the electromagnetic induction heating, onlythe interface (adhesion surface) between the steel sheet and the filmmay sufficiently be heated to improve the adhesion property of the film.Furthermore, when the primer is used, the primer is heated from theinside (steel sheet side) and the solvent residual quantity iscomparatively small after heating.

In the method of the invention, the fluororesin film-coated steel sheetis then cooled by slow cooling in air or quenching with water, and theinfluence upon the adhesion strength can be ignored irrespective ofcooling methods.

In the method of the invention, galvanized steel sheet, hot dip zinc-5%aluminum alloy plated steel sheet, hot dip galvannealed steel sheet,zinc-nickel alloy plated steel sheet, hot dip aluminum plated steelsheet, hot dip aluminum-silicon alloy plated steel sheet, 55%aluminum-zinc alloy plated steel sheet, stainless steel sheet or thelike is preferably used as the steel sheet substrate.

These steel sheets are favorable to have a thickness of about 0.2-1.6mm, while the type thereof is not particularly restricted and isselected in accordance with use purpose. For instance, aluminum platedsteel sheet and stainless steel sheet are suitable to the applicationsrequiring high thermal resistance such as a top plate for a gas cookingstove and the like.

Furthermore, in order to obtain a high film adhesion property in case offluororesin film having a high melting point such as PFA, the steelsheet surface is heated to be a high temperature of about 400° C. sothat the galvanized steel sheet and the like are not suitable for such arequirement because the plated layer is molten.

As the fluororesin film covering the above steel sheet material in themethod of the invention, fluororesin films containing 2 or more fluorineatoms per 1 molecule unit and having a poor adhesion property, whichinclude, for example, films of thermoplastic resins such as ethylenetetrafluoride-perfluoroalkylvinyl ether copolymer resin (PFA), ethylenetetrafluoride-propylene hexafluoride copolymer resin (FEP) and ethylenetetrafluoride-ethylene copolymer resin (ETFE) are effectively used.

The thickness of the fluororesin film is not particularly restricted,but is preferably 10-100 μm.

Moreover, these films are naturally colorless and transparent, but theremay similarly be utilized films subjected to any treatment, e.g. filmcontaining pigment powder for coloring, film containing additives forimparting functionality and the like.

In the method of the invention, if necessary, an under coat treatment ofa metal is carried out prior to the adjustment of the steel sheetsurface by heating. The under coat treatment is desirable to be carriedout by the application of the primer.

In addition to the application of the primer, the under coat treatmentmay be conducted by the other pretreatment such as a general method inwhich the steel sheet is polished by shot burst or wire blushing andthen subjected to pickling or electrolytic etching, or a roll-on typechromate treatment. In case of the stainless steel sheet, when the sheetis subjected to the roll-on type chromate treatment and further primertreatment instead of the mere primer treatment, the film adhesionstrength, thermal resistance and durability are considerably improved.

As the primer for the under coating the steel sheet matrix, use may bepreferably made of aqueous solution or organic solution of primers eachcomprised of a mixed resin of fluororesin powder selected from the groupconsisting of polyethylene tetrafluoride (PTFE), ethylenetetrafluoride-perfluoroalkylvinyl ether copolymer resin (PFA), ethylenetetrafluoride-propylene hexafluoride copolymer resin (FEP), ethylenetetrafluoride-ethylene copolymer resin (ETFE), chlorotrifluoroethylene,polyvinylidene fluoride and the like and a heat-resistant resin selectedfrom the group consisting of polyether sulfone (PES), polyphenylenesulfide (PPS), polyimide, polyamideimide, epoxy resin, silicone resin,phenolic resin and the like.

The mixing ratio of the fluororesin powder to heat-resistant resin ispreferably within a range of 1:99-95:5 by weight ratio.

The thickness of the primer is favorably 3-15 g/m² after the drying.When it is less than 3 g/m², the effect on improving the adhesionstrength as the primer is poor, while when it exceeds 15 g/m², theeffect is saturated and also there is a possibility of lowering the filmadhesion property in the working or of creating blister of the primerdue to remaining gas in the primer.

The examples according to the invention will be described as comparedwith comparative examples.

EXAMPLE 1

A fluororesin film is heat-fused to each of steel sheets treated withprimer having different resin compositions shown in Table 1 undervarious conditions to produce a steel sheet coated with the fluororesinfilm (see FIG. 1).

The primary adhesion property of the thus fluororesin film coated steelsheet are evaluated to obtain results as shown in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    Under coating        Heat-fusing                                              conditions           conditions (°C.)                                  Roll-on              Heating                                                                             Temperature                                                                         Film                                               type       Resin                                                                             temperature                                                                         of roll                                                                             Melting                                        Material                                                                          chromate   baking                                                                            of steel                                                                            facing                                                                              point of                                                                          Kind                                       Kind                                                                              treatment                                                                          Resin method                                                                            sheet the film                                                                            film                                                                              of                                       No                                                                              *1  *2   composition                                                                         *3  (°C.)                                                                        (°C.)                                                                        (°C.)                                                                      film                                                                             Remarks                               __________________________________________________________________________    1 1   absence                                                                            PFA:1 A   340   208   308 PFA                                                                              Invention                             2 1   presence                                                                           PES:2 A   380   120   270 FEP                                                                              Example                               3 2   absence                                                                            PPS:1 A + B                                                                             380   180   308 PFA                                      4 3   absence    A   400   170   308 PFA                                      5 3   presence                                                                           FEP:1 A   390   130   308 PFA                                      6 3   presence                                                                           PES:2 A   420   108   308 PFA                                      7 3   presence                                                                           --    A   360   165   270 FEP                                      8 3   presence                                                                           --    A   360   150   308 PFA                                      9 3   presence                                                                           --    B   350   160   308 PFA                                      10                                                                              3   presence                                                                           --    B   330   160   270 FEP                                      11                                                                              1   absence                                                                            PFA:1 A   320   260   308 PFA                                                                              Comparative                           12                                                                              2   absence                                                                            PES:2 A   390   240   308 PFA                                                                              Example                               13                                                                              3   presence                                                                           PPS:1 A   290   130   270 FEP                                      14                                                                              3   presence                                                                           --    B   450    80   308 PFA                                      15                                                                              3   presence                                                                           --    B   380    40   308 PFA                                      16                                                                              3   presence                                                                           --    B   410   110   265 ETFE                                     __________________________________________________________________________     *1: 1: 55% Al--Zn plated steel sheet, thickness 0.6 mm, plated amount         AZ150 2: hotdip Al--Si alloy plated steel, thickness 0.6 mm, plated amoun     40 g/m.sup.2 (oneside) 3: SUS430 stainless steel sheet, thickness 0.6 mm      *2: Rollon type chromate solution ZMDS-1300AN, made by Japan Parkerizing      Co., Ltd.                                                                     *3: A: induction heating B: hot air heating                              

                  TABLE 2                                                         ______________________________________                                                 Film adhesion                                                        No.      property           Remarks                                           ______________________________________                                        1        5                  Invention                                         2        5                  Example                                           3        5                                                                    4        5                                                                    5        5                                                                    6        5                                                                    7        5                                                                    8        5                                                                    9        4                                                                    10       4                                                                    11       2                  Compar-                                           12          4 * 1           ative                                             13       1                  Example                                           14          3 * 1                                                             15       1                                                                    16       2                                                                    ______________________________________                                         * 1: Melt breakage of film is created in the adhesion of film. The            production stability is less.                                            

Moreover, as the heat pressing conditions of the fluororesin film in No.1-10 and No. 11-16 of Example 1, the temperature of the press rollfacing the steel sheet is 150°-250° C. and line pressure of the roll is0.3 kg/cm. And also, the roll-on type chromate treating weight is 30mg/m² and the chromate coating is dried at 150° C. for 30 seconds.

Further, the test method and evaluating method for the adhesion propertyin the above example are conducted under the following conditions.

Test for adhesion property!

It is carried out according to #-type Erichsen test method of JIS Z6744.Three test specimens used in the #-type Erichsen test method areprovided by previously immersing two specimens in a boiling water for 1or 2 hours and then drying in air for 30 minutes or without immersion inthe boiling water. Each of #-type portion of these specimens is pushedout by 6 mm with Erichsen testing machine, and then the cut surface ofthe pushed film portion is floated by means of a knife and the film isforcedly peeled off therefrom by means of tweezers.

The evaluation standard is shown in Table 3, in which the case of notfloating all of three specimens is 5 evaluation point.

                  TABLE 3                                                         ______________________________________                                                        Immersing time in boiling water                                               No immersion  1 hour                                                                              2 hours                                   ______________________________________                                        Evaluation                                                                              5     ∘ ∘                                                                       ∘                             point     4     ∘ ∘                                                                       Δ                                             3     ∘ Δ                                                                             x                                                   2     ∘ x     x                                                   1     x             x     x                                         ______________________________________                                         ∘: No float of film                                               Δ: Film is floated but can not be peeled off                            x: Film is floated and peeled off                                        

As seen from the results of Table 2, all of the fluororesin film coatedsteel sheets produced by the method of the invention are excellent inthe film adhesion property as compared with those of the fluororesinfilm coated steel sheets according to the comparative examples.Furthermore, according to the method of the invention, the fluororesinfilm coated steel sheets can be produced continuously without creatingthe melt breakage of the film in the pressing of the resin film.

INDUSTRIAL APPLICABILITY

As mentioned above, according to the method of the invention, thefluororesin film containing 2 or more fluorine atoms per 1 molecule unitcan be adhered strongly onto the surface of the steel sheet, and alsothe fluororesin film coated steel sheets having an excellent adhesionproperty can be produced continuously and economically.

Therefore, the fluororesin film coated steel sheets according to theinvention are applicable to not only materials for cooking tools,kitchen tools, an inner plate of a microwave oven and a top plate of agas table but also materials for electron parts, printed circuit boardsand the like and can be supplied as a cheap and excellent materialsuitable for wide applications.

We claim:
 1. A method of producing a steel sheet coated with afluororesin film by heat-fusing the fluororesin film onto a surface ofthe steel sheet, comprising:heating the surface of the steel sheet to atemperature higher by 30°-120° C. than a melting point of thefluororesin of said fluororesin film; and contact bonding thefluororesin film onto the steel sheet surface between a pair of rolls,while a surface of a roll facing the fluororesin film, in said pair ofrolls, is heated to a temperature lower by 100°-210° C. than the meltingpoint of the fluororesin of the fluororesin film.
 2. A method ofproducing a steel sheet coated with a fluororesin film according toclaim 1, wherein a primer comprised of a mixed resin of fluororesin andthermal-resistant resin is applied onto the surface of the steel sheetand dried, prior to contact-bonding of the fluororesin film to the steelsheet, the primer having a thickness of 3-15 g/m² after drying.
 3. Amethod of producing a steel sheet coated with a fluororesin filmaccording to claim 1, wherein at least one of the steel sheet surfaceand the roll surface facing the fluororesin film is heated usinginduction heating.
 4. A method of producing a steel sheet coated with afluororesin film according to claim 2, wherein at least one of theprimer which has been applied to the steel sheet surface and the rollsurface facing the fluororesin film is heated using induction heating.5. A method of producing a steel sheet coated with a fluororesin filmaccording to claim 1, wherein the steel sheet comprises a memberselected from the group consisting of galvanized steel sheet, hot dipzinc-5% aluminum alloy plated steel sheet, hot dip galvannealed steelsheet, zinc-nickel alloy plated steel sheet, hot dip aluminum platedsteel sheet, hot dip aluminum-silicon alloy plated steel sheet, 55%aluminum-zinc alloy plated steel sheet, and stainless steel sheet.
 6. Amethod of producing a steel sheet coated with a fluororesin filmaccording to claim 5, wherein the thickness of the steel sheet is fromabout 0.5 to about 1.6 mm.
 7. A method of producing a steel sheet coatedwith a fluororesin film according to claim 1, wherein the fluororesinfilm comprises a fluororesin containing 2 or more fluorine atoms per 1molecule unit.
 8. A method of producing a steel sheet coated with afluororesin film according to claim 7, wherein the fluororesin filmcomprises a member selected from the group consisting of ethylenetetrafluoride-perfluoroalkylvinyl ether copolymer resin, ethylenetetrafluoride-propylene hexafluoride copolymer resin and ethylenetetrafluoride-ethylene copolymer resin.
 9. A method of producing a steelsheet coated with a fluororesin film according to claim 7, wherein thethickness of the fluororesin film is from 10 to 100 μm.
 10. A method ofproducing a steel sheet coated with a fluororesin film according toclaim 2, wherein the primer comprises a mixed resin comprising afluororesin powder and a thermal-resistant resin and wherein:A) thefluororesin powder comprises a member selected from the group consistingof polyethylene tetrafluoride, ethylenetetrafluoride-perfluoroalkylvinyl ether copolymer resin, ethylenetetrafluoride-propylene hexafluoride copolymer resin, ethylenetetrafluoride-ethylene copolymer resin, chlorotrifluoroethylene, andpolyvinylidene fluoride; and B) the thermal-resistant resin comprises aheat-resistant resin comprising a member selected from the groupconsisting of polyether sulfone, polyphenylene sulfide, polyimide,polyamideimide, epoxy resin, silicone resin and phenolic resin.
 11. Amethod of producing a steel sheet coated with a fluororesin filmaccording to claim 10, wherein a mixing ratio of the fluororesin powderto heat-resistant resin is in the range of from 1:99 to 95:5, by weight.